This interdisciplinary study is aimed at obtaining a better understanding of the mechanisms by which ultrasound interacts with tissue and induces biological effects. Using physical theory as a guide, we plan to determine those conditions under which biological damage would be most likely to result from low intensity ultrasound exposure. Physical theory will be extended to include non-linear interactions between ultrasound and tissue. Empirical studies will include observation of alveolar perturbations due to ultrasonic irradiation of excised hamster lungs, the effect of the presence of gas bubbles on thresholds for biological effects, and the predicted generation of audible sound when pulsed ultrasound interacts with tissue. An important benefit of this research is that the results can serve as a guide for indicating what to look for in the search for harmful effects arising out of clinical ultrasound usage, and for indicating under which conditions these effects can be avoided.